In a cellular communications network, a user equipment device can establish a radio link with a base station that provides coverage to a particular cell. As the user equipment device moves through the network coverage area, from one cell to another, a handover is performed, so that the user equipment (UE) establishes a link with a second base station and breaks the link with the first base station.
Cellular communications network have well-established procedures for performing handovers, as defined in the relevant standards documents. These procedures depend on various parameters, which can be set by the network operator. Setting the handover parameters typically involves setting values for various radio link quality metrics, such that a handover is initiated when the parameter values are reached.
For example, a handover from a source cell to a target cell will typically rely on quality metrics of the radio link between the UE and the source cell and the radio link between the UE and one or more candidate target cells. The quality metrics are typically derived from measurements performed by the UE on downlink reference signals and reported back to the serving cell. The serving cell typically broadcasts the quality metrics that the UE is required to measure, and the thresholds that define the events that the UE is required to report back to the serving cell. For example, in the case of a 3rd Generation Partnership Project (3GPP) Wideband Code Division Multiple Access (WCDMA) or High Speed Packet Access (HSPA) network the quality metrics might be the Common Pilot Channel (CPICH) Received Signal Code Power (RSCP). In the case of a 3GPP Long Term Evolution (LTE) network the quality metrics might be the CPICH chip power to noise ratio (Ec/No) and Reference Signal Receive Power (RSRP) and/or Reference Signal Receive Quality (RSRQ). The serving cell then uses these UE measurements as inputs to vendor specific algorithms that control initiation and cancellation of handover procedures.
Thus, the first stage of a possible handover is that reporting of measurements for neighbouring cells is initiated if one or more quality metrics of one or more neighbouring cells exceeds either an absolute threshold (e.g. event 1E in WCDMA/HSPA or event A4 in LTE) or a threshold relative to the serving cell (e.g. event 1C in WCDMA/HSPA or event A3 in LTE).
Handover to a neighbouring cell is then typically triggered only if the quality metrics for the neighbouring cell are above a defined minimum absolute threshold and the quality metrics for the serving cell are below a defined minimum absolute threshold, or if the quality metrics for the neighbour cell are above the quality metrics of the serving cell.
However, it is known that a UE may encounter propagation environments where the quality of the link from the serving cell deteriorates rapidly and the quality of the link towards a neighbouring cell improves rapidly. This can mean that there is insufficient time for the UE to complete the handover preparation phase with the serving cell before the radio link to the serving cell is lost. Thus, in such an environment, the handover procedure is unreliable, leading to a high rate of handover failures.
This problem is well known, and can be mitigated through manual optimisation of the network which typically involves: detecting problem areas through monitoring handover key performance indicators (KPIs), performing drive tests or site scans in the identified problem areas, analysing handover traces associated with the failed handovers, and tuning the handover parameters and re-iterating the process until KPI targets are met. This is a time-consuming and expensive process for the network operator.
The handover parameters can be tuned such that handovers are initiated more readily, for example by setting short values for timer parameters and quality metrics thresholds that are readily met. However, setting such values for all target cells or throughout the network could cause additional unwanted signalling into the core network as the UE would respond to small changes in its RF environment.